If you check the "price" for each hosting account, it seems to be using a points system here giving you 1 point for each post, or something to that affect. So it is my understanding, you will need to post a minimum of your hosting package's cost in posts, each month.
So the basic beginner hosting plan, costs 10 posts, so you need to make at least 10 posts per month to stay active.
Admin can correct me if there's more to it or I have made an error, but I'm pretty sure that's how it works.

As long as staying active is concerned... I believe in a community, it makes sense to talk sense, and not force to talk junk. So well, there is no minimum number of posts that you need to make every day, week or month. But as soon as I see no activity in the forums, the ones who haven't been on the forum for the longest period, while holding a hosting account, will be the first ones to get their accounts suspended.